Display device and liquid crystal display panel

ABSTRACT

A display device is provided that comprises a liquid crystal display panel for displaying an image by spatial light modulation, and circuitry for switching liquid crystal in the panel between having a first configuration in a first mode to cause an image displayed using the panel to be discernible from a wide range of viewing angles, and having a second configuration in a second mode to cause an image displayed using the panel to be discernible substantially only from within a narrow range of viewing angles. Several types of display panel to achieve such in-panel switching between public and private viewing modes are disclosed.

This Nonprovisional application is a divisional of Nonprovisional patentapplication Ser. No. 11/428,883 filed Jul. 6, 2006 now U.S. Pat. No.7,965,268, which claims priority under 35 U.S.C. §119(a) on PatentApplications No. 0513971.2 and No. 0513968.8 filed in U.K. on Jul. 8,2005, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a display device and a liquid crystaldisplay panel for use in a display device.

BACKGROUND OF THE INVENTION

Electronic display devices, such as monitors used with computers andscreens built in to telephones and portable information devices, areusually designed to have a viewing angle as wide as possible, so thatthey can be read from as many viewing positions as possible.

However, there are some situations where it is useful to have a displaythat is visible from only a narrow range of angles. For example, where aperson is reading a confidential or private document on the display of amobile device in a crowded place, he would wish to minimise the risk ofothers around him also having sight of the document on the display.

It is therefore useful to have a display device that is switchablebetween two modes of operation. In a ‘public’ mode, the display devicewould have a wide viewing angle for general use. In a ‘private’ mode,the display device would have a narrow viewing angle, so that privateinformation could be read in a public place.

For example, when certain secure web pages are accessed (e.g. bank siteweb pages), or when a certain PIN (personal identification number) isinput to the keyboard (e.g. bank account PIN), the display couldautomatically go into the privacy mode. In the private mode, anindicator or icon could be shown on the screen to indicate that theprivate mode is active.

This concept can be applied to many other types of devices where a usermay which to view confidential information, but cannot control who elsemay be watching. Examples are mobile phones, Personal Digital Assistants(PDAs), laptop PCs, desktop monitors, Automatic Teller Machines (ATMs)and Electronic Point of Sale (EPoS) equipment.

A number of devices are known which restrict the range of angles orpositions from which a display can be viewed.

U.S. Pat. No. 6,552,850 describes a method for the display of privateinformation on a cash dispensing machine. Light emitted by the machine'sdisplay has a fixed polarisation state. The machine and its user aresurrounded by a large screen of sheet polariser that absorbs light ofthat polarisation state but transmits the orthogonal state. Passers-bycan see the user and the machine but cannot see information displayed onthe screen.

One method for controlling the direction of light is the use of a“louvred” film. Such a film consists of alternating transparent andopaque layers in an arrangement similar to a Venetian blind. Theselayers may be perpendicular to the surface of the film or at some otherangle. Like a Venetian blind, it allows light to pass through it whenthe light is travelling in a direction nearly parallel to the plane ofthe layers, but absorbs light travelling at large angles to the plane ofthe layers. Methods for the production of such films are described inU.S. RE 27,617, U.S. Pat. No. 4,766,023 and U.S. Pat. No. 4,764,410.

Other methods exist for making films with similar properties to thelouvred film. These are described, for example, in U.S. Pat. No.5,147,716 and U.S. Pat. No. 5,528,319.

The techniques described above may be used to restrict the range ofangles from which a display can be viewed; in other words, they can beused to make a display ‘private’. However none of them gives a method bywhich the privacy function can easily be switched off to allow viewingfrom a wide range of angles.

Several methods are known for providing a display that can be switchedbetween a public mode (with a wide viewing angle) and a private mode(with a narrow viewing angle).

US 2002/0158967 describes the use of a light control film mounted on adisplay so that the light control film can be moved over the front ofthe display to give a private mode, or mechanically retracted into aholder behind or beside the display to give a public mode. This methodhas the disadvantages that it requires moving parts that may fail or bedamaged, and it adds significant bulk to the display.

One method for switching from public to private mode with no movingparts is to mount a light control film behind the display panel, and toplace a diffuser that can be electronically switched on and off betweenthe light control film and the panel. When the diffuser is inactive, thelight control film restricts the range of viewing angles and the displayis in the private mode. When the diffuser is switched on, it causeslight travelling at a wide range of angles to pass through the panel andthe display is in the public mode. It is also possible to mount thelight control film in front of the panel and place the switchablediffuser in front of the light control film to achieve the same effect.

Switchable privacy devices of this type are described in U.S. Pat. No.5,831,698, U.S. Pat. No. 6,211,930 and U.S. Pat. No. 5,877,829. Theyshare the disadvantage that the light control film absorbs a significantfraction of the light incident upon it, whether the display is in thepublic or the private mode. The display is therefore inefficient in itsuse of light. Since the diffuser spreads light through a wide range ofangles in the public mode, these displays are also dimmer in the publicmode than in the private mode, unless the backlight is made brighter tocompensate.

Another disadvantage relates to the power consumption of such devices.In the public mode of operation, the diffuser is switched off. Thiswould typically mean that a voltage is applied to a switchablepolymer-dispersed liquid crystal diffuser. More power is thereforeconsumed in the public mode than in the private mode. This is adisadvantage for displays that are used for most of the time in thepublic mode.

Another known method for providing a switchable public/private displayis described in U.S. Pat. No. 5,825,436. The light control devicedisclosed is similar in structure to the louvred film described above.However, each opaque element in the louvred film is replaced by a liquidcrystal cell that can be electronically switched from an opaque state toa transparent state. The light control device is placed in front of orbehind a display panel. When the cells are opaque, the display is in aprivate mode; when the cells are transparent, the display is in a publicmode.

One disadvantage of this method relates to the difficulty and expense ofmanufacturing liquid crystal cells with an appropriate shape. Anotherdisadvantage is that, in the private mode, a ray of light may enter atan angle such that it passes first through the transparent material andthen through part of a liquid crystal cell. Such a ray will not becompletely absorbed by the liquid crystal cell and this may reduce theprivacy of the device.

Another method for producing a switchable public/private display deviceis disclosed in JP 3607272. The disclosed device uses an additionalliquid crystal panel, which has patterned liquid crystal alignment.Different aligned segments of the panel modify the viewingcharacteristics of different areas of the display in different ways,with the result that the whole display panel is fully readable only froma central position.

GB-A-2405544 and JP 2005-078093 describe switchable privacy devicesbased on louvres, which operate only for one polarisation of light. Thelouvres are switched on and off either by rotating dyed liquid crystalmolecules in the louvre itself or by rotating the plane of polarisationof the incident light using a separate element.

GB-A-2410116 (WO 2005/071449) disclose various backlight arrangementsfor use in a display device having the ability to switch the viewingangle between public and private modes, for example. Further knownsystems and techniques in this area are also described therein.

GB-A-2413394 (US 2005/0243265) discloses a switchable privacy devicethat is constructed by adding one or more extra liquid crystal layersand polarisers to a display panel. The intrinsic viewing angledependence of these extra elements can be changed by electricallyswitching the liquid crystal.

US 2003/0146893 discloses a polarisation modifying layer (PML) that isplaced behind the exit polariser of a liquid crystal display panel. Someparts of the PML are transparent. Other parts change the polarisation oflight passing through them so that pixels viewed through these parts areinverted in colour (bright pixels becoming dark and dark pixels becomingbright). Data sent to pixels directly behind these parts are inverted sothat when the display is viewed from a central position, the imageappears normally. However, when the display is viewed from a differentangle, different pixels are viewed through the retarder elements and theimage is corrupted. Off-axis viewers see a confusing image, for examplea random dot pattern. The PML may be made from liquid crystal andswitched off to give a public mode.

GB-A-2418518 discloses a device in which a guest host (dyed) LC layerwith a patterned electrode is added to a standard TFT LC display. Thedyed LC layer can be switched between an absorbing (private) andnon-absorbing state (public). The dye molecules absorption is dependentupon the incident angle and polarisation of light. For a givenpolarisation and orientation the absorption of the dye increases withlarger viewing angles resulting in low brightness at high angles (narrowmode).

Co-pending British Patent Application No. 0510422.9 discloses thecombination of a privacy function and a 3D function provided by a singleadditional switch cell. The display has three operating states: a widemode; a private mode; and a 3D mode. Both patterned and unpatterned LCalignment examples are described.

The concept of using a hologram to provide a privacy function which wasfirst described in GB-A-2404991 (US 2005/0063029). However, due tounwanted diffraction of light from the display by the hologram, thecolour of the image seen by viewers may be affected. Furthermore, forapplications using a touch screen mounted on the front of the display,the user's hand can block the illumination of the hologram and so reducethe effectiveness of the privacy mode.

Co-pending British Patent Application No. 0511536.5 discloses the use ofan extra liquid crystal layer located between the existing polarisers ofan LCD panel. In this location the extra switch cell can modify thegreyscale curves for off-axis light. This provides a higher level ofprivacy for images than the techniques disclosed in, for example,GB-A-2413394 (US 2005/0243265).

U.S. Pat. No. 5,109,219 describes a method for controlling the viewingangle of a LC display by converting a digital view angle parameter to ananalogue bias voltage which is applied to the LC. However, thistechnique will only serve to modify the view angle characteristics ofthe display, and will not tend to hide the image at wide angles.

U.S. Pat. No. 5,936,596 and JP 2003-295160 (US 2006/0126156) describechanging the voltage range applied to the pixels in an LC display tochange the viewing angle. Look-up tables are used to change the displaybetween narrow and wide view-angle modes. However, this method does notconceal displayed information as such when in the narrow mode, it onlymodifies the grey-scale mapping to distort the image.

The article “A Method for Concealment of Displayed Data”, M. Dogruel,Displays, vol. 24, no. 3, October 2003, describes a method forconcealing data shown on a display by time-sequentially rendering theimage and its inverse at a rate faster than the human eye can perceive.The eye of a casual observer thus averages the images and therefore seesa uniform grey display screen. To see the private image, the user mustwear shuttered glasses synchronised with the display, such that theinverse image is blocked. This method has a number of drawbacks:firstly, the user must wear shuttered glasses in order to observe thecorrect image; secondly, image privacy can also be compromised byrapidly moving a toothed object across the view of the display and thusobscuring some parts of the cancelling image; and thirdly a ghost imagecan be observed as it is very difficult to design the two images tocancel perfectly. This article also describes adding a third image toact as a confusing image, but this requires the display to run at threetimes the normal video rate.

Rocket Software, Inc. (http://www.rocketsoftware.com) have developed asoftware package that provides some level of privacy using the inherentproperties of an LC display. The software modifies the image sent to thedisplay by applying an extra patterning across the whole image thatreduces the grey levels or contrast of the image. Due to the non-linearresponse of the display, the level of reduction is such that, whenviewed on-axis, the image is only slightly disturbed but, when viewedoff-axis, the non-linear response of the display leads to an enhancedcontrast patterning. However, this solution does inevitably affect theon-axis performance of the display in some degree, and the patternvisibility will disturb even the authorised user when using the displayin the private mode. Further, in practice, the patterning is notsufficient to provide an adequate level of privacy off-axis.

WO 03/015424 discloses a light switching apparatus that comprises apassive birefringent lens and a switchable polariser. By switching thepolarisation, different directional distributions of output light areprovided. However, when activated, the lenses do not discriminate inangle which light is imaged.

U.S. Pat. No. 6,369,949 discloses an optically anisotropic micro-lenswindow. The imaging element described is not switchable, andconsequently a device making use of this technology could not beswitchable between public and private modes of operation.

GB-A-2410339 discloses the use of multiple arrays of polarisationsensitive lenses in a polarisation optical conversion system.

JP 09-230377 and U.S. Pat. No. 5,844,640 describe a method of changingthe viewing angle properties of a single layer LCD panel. This isachieved for a Vertically Aligned Nematic (VAN) LC mode. Electric fieldsin the plane of the display panel are used to control how the LCmaterial tilts in a pixel area. The number and orientation of differenttilt domains within a pixel can be controlled by the in-plane fields. Apixel with several tilt domains will have a wide viewing angle, while apixel with one tilt domain will have a narrower viewing angle. The useof such a method to vary the viewing angle of a display is described.However, the viewing angle of a single tilt domain of the VAN modedescribed is generally not sufficiently narrow to provide a good privacymode.

JP 3405972 describes a single LC panel which uses patterned LC alignmentto provide a narrow viewing angle mode LCD. However, this narrow mode isfixed, and there is no wide viewing mode.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda display device comprising a liquid crystal display panel fordisplaying an image by spatial light modulation, and circuitry forswitching liquid crystal in the panel between having a firstconfiguration in a first mode to cause an image displayed using thepanel to be discernible from a wide range of viewing angles, and havinga second configuration in a second mode to cause an image displayedusing the panel to be discernible substantially only from within anarrow range of viewing angles.

The second configuration of liquid crystal may cause an image-confusingpattern to be visible in the image discerned by a viewer outside thenarrow range of angles.

The liquid crystal in the first configuration may comprise a singlearrangement of liquid crystal across the display device.

The liquid crystal in the first configuration may comprise a pluralityof lateral regions each having one of at least two differentarrangements of liquid crystal

The first configuration regions may be sized so as not to be resolvableby a viewer.

The liquid crystal in the second configuration may comprise a pluralityof lateral regions each having one of at least two differentarrangements of liquid crystal.

The second configuration regions may be sized so as to be resolvable bya viewer.

The second configuration regions may have a lateral dimension at leasttwo times greater than a lateral dimension of a picture element of thepanel.

The second configuration regions may have a lateral dimension at leastfive times greater than a lateral dimension of a picture element of thepanel.

The second configuration regions may have a lateral dimension at leastten times greater than a lateral dimension of a picture element of thepanel.

The second configuration regions having the same or similar liquidcrystal arrangement may be arranged spatially in a predetermined manner.

The second configuration regions of the same or similar liquid crystalarrangement may be arranged spatially in a chequerboard pattern or apattern of text or a logo.

The circuitry may comprise a plurality of in-plane electrodes forswitching the liquid crystal to the second configuration.

The circuitry may comprise in-plane electrodes disposed at least to orat or towards each side of each second configuration region.

The circuitry may comprise three or more in-plane electrodes disposedwithin each second configuration region.

The in-plane electrodes may be patterned in at least two differentorientations to produce the at least two different arrangements.

The electrodes may be disposed on the same side of the liquid crystaldisplay panel as electrodes used for switching picture elements of thepanel.

Neighbouring regions may be arranged to have different arrangements ofliquid crystal.

The at least two arrangements may comprise liquid crystal havingdifferent respective substantially uniform orientations of liquidcrystal.

The at least two arrangements may comprise one or more pairs oforientations, the orientations in the or each pair being disposedsubstantially symmetrically about a predetermined axis.

The predetermined axis may lie in the narrow range of viewing angles.

The at least two different arrangements of liquid crystal may havedifferent respective angular transmission functions.

The respective angular transmission functions may be asymmetric about anaxis lying within the narrow range of viewing angles.

The respective angular transmission functions for the secondconfiguration may be substantially equal for viewing angles within thenarrow range, and different for viewing angles outside the narrow range.

The respective angular transmission functions for the firstconfiguration may be spatially averaged by a viewer in the first mode toprovide a smoothly-varying average transmission function across at leastpart of the wide range of viewing angles.

The average transmission function may vary smoothly across the whole ofthe wide range of viewing angles.

The first and second configurations may be vertically aligned nematicconfigurations.

The first and second configurations may be bi- or multi-stable liquidcrystal states, and the circuitry may be adapted to switch the liquidcrystal between these states.

The display device may comprise an alignment layer for producing the bi-or multi-stable states.

The first configuration may be a continuous pinwheel alignedconfiguration.

An image may be represented by a plurality of image elements, and thedisplay device may comprise means for modifying the respective datavalues of at least some of the image elements such that when themodified image is displayed in a first scenario using a display panelhaving a first data value-to-luminance response to a viewer, the imageperceived by the viewer through spatial averaging is substantially thesame as the original image, and such that when the modified image isdisplayed in a second scenario using a display panel having a seconddata value-to-luminance response to a viewer, different to the firstdata value-to-luminance response, the image perceived by the viewerthrough spatial averaging is different to the original image, andwherein the first and second configurations of liquid crystal arearranged to provide the display panel with substantially the first andsecond data value-to-luminance responses respectively for viewing anglesoutside the narrow range, and are both arranged to provide substantiallythe first data value-to-luminance response for viewing angles inside thenarrow range.

The second data value-to-luminance response may be a non-linear datavalue-to-luminance response.

The first data value-to-luminance response may be a substantially lineardata value-to-luminance response.

The first configuration regions may be adapted such that lighttravelling at an angle outside the narrow range of angles passes throughat least two regions having different arrangements of liquid crystal soas to have the first data value-to-luminance response.

The second configuration regions may be adapted such that lighttravelling at an angle outside the narrow range of angles has the seconddata value-to-luminance response.

The first and second configurations may be twisted nematicconfigurations.

The display device may comprise at least one patterned alignment layerfor producing the first and second configurations.

The circuitry may be operable to apply an electric field to change thealignment properties of the at least one alignment layer to switch theliquid crystal between the first and second configurations.

The circuitry may be operable to apply an electric field across and/orin the plane of the liquid crystal to switch the liquid crystal betweenthe first and second configurations.

The circuitry may be operable to apply fringe electric fields.

The original image may be substantially hidden in the image perceived inthe second scenario.

At least some of the data values may be modified in dependence upon amasking image.

Each data value may be modified in dependence upon the data value at acorresponding position of the masking image.

The image perceived in the second scenario may resemble at least to someextent the masking image.

The masking image may be such as to provide a high degree of visuallyconfusing information in the second scenario.

The masking image may comprise a chequerboard pattern or a pattern oftext or a logo.

Different masking images may be used in different time frames.

At least some of the data values may be modified in dependence upon amasking parameter.

The degree of modification may be determined at least in part by themasking parameter.

The data values may be modified such that localised groups of displayedimage elements are perceived in the first scenario through spatialaveraging to have substantially the same overall luminance as thoseimage elements would have done without such modification.

The degree of modification for each image element in a group may bedetermined in dependence upon the data value at a position in themasking image corresponding to the image element.

If any modification is to be performed for a group, the data value of atleast one image element in the group may be increased while the datavalue of at least one other image element in the group may be decreased.

The amount of increase may be substantially the same as the decrease.

The amount of increase relative to the amount of decrease may bedetermined in dependence upon the first data value-to-luminanceresponse.

The image elements designated for increase and decrease may be swappedin different time frames.

For each of the at least one image element an amount related to thecorresponding respective masking image data value may be added to theimage element data value, and for each of the at least other one imageelement an amount related to the corresponding respective masking imagedata value may be subtracted from the image element data value.

The amount may be equal to the corresponding masking image data value.

The amount may be determined in dependence upon the difference betweenthe image data value and the maximum or minimum data value, whichever iscloser.

The amount may be proportional to the difference multiplied by thecorresponding masking image data value.

Each group may comprise two image elements.

The data values of at least some of the image elements may be averagedin a group.

The data values of at least some of the masking image elements may beaveraged in a group.

Image elements having corresponding modifications performed on them maybe arranged in lines of the image.

Image elements having corresponding modifications performed on them maybe arranged in columns of the image.

Image elements having corresponding modifications performed on them maybe arranged in a chequerboard pattern or a pattern of text or a logo.

It may be ensured that a modified data value does not fall outside thenormal range of data values allowed.

The data value range of the image may be compressed before modification.

The data value range of the masking image may be compressed beforemodification.

It may be that only the data values of the image elements in asub-portion of the image are so modified.

Each image element may relate to a plurality of colour componentscorresponding to a pixel of the display device.

Each image element may relate to a single colour component correspondingto a sub-pixel of the display device.

The viewer may be at least a predetermined distance away from thedisplay device.

The circuitry may be adapted to operate the display panel to apply anelectric field in first and second different ways in the first andsecond modes respectively to achieve the first and second liquid crystalconfigurations.

The circuitry may be adapted to operate the display panel usingdifferent respective ranges of applied electric field strengths in thefirst and second modes.

The circuitry may be adapted to operate the display panel usingdifferent respective directions of applied electric field in the firstand second modes.

The circuitry may be adapted to operate the display panel using in-planeswitching in the first mode and electrically controlled birefringenceswitching in the second mode.

The circuitry may comprise a first set of electrodes for performing thein-plane switching and a second set of electrodes for performing theelectrically controlled birefringence switching.

The first set electrodes may be disposed on the same side of the displaypanel to apply a field substantially in the plane of the panel.

The second set electrodes may be disposed on opposite sides of thedisplay panel to apply a field across the panel.

The display panel may comprise first and second sets of regions, and thecircuitry may be adapted to apply the electric field in the first andsecond ways in the first and second sets of regions respectively.

Pixels of the display may each comprise a region from the first set anda region from the second set.

The circuitry may be operable in the second mode to operate some pixelsin the first way and other pixels in the second way so as to cause animage confusing pattern to a viewer outside the narrow range of viewingangles.

Pixels of the display may each comprise first and second regions havingliquid crystal respectively having the first and second configurations.

The circuitry may be operable in the second mode to operate some pixelsusing the first regions and other pixels using the second regions so asto cause an image confusing pattern to a viewer outside the narrow rangeof viewing angles.

The circuitry may be adapted to operate in the second mode to usedifferent respective driving voltage ranges to produce the at least twodifferent arrangements of liquid crystal, the liquid crystalarrangements being such as to have substantially the same transmissionto viewers within the narrow range of angles for different respectivevoltages selected from each of the driving voltage ranges and differentrespective transmissions for those selected voltages to viewers outsidethe narrow range of angles.

A first one of the driving voltage ranges may have atransmission-to-voltage function suitable for image display to viewersoutside the narrow range of angles and a second one of the drivingvoltage ranges may have a transmission-to-voltage function unsuitablefor image display to viewers outside the narrow range of angles.

The second one of the driving voltage ranges may have a substantiallyconstant, low, transmission to viewers outside the narrow range ofangles for voltages across at least most of the range.

The substantially constant, low, transmission may be a substantiallyzero transmission.

The second one of the driving voltage ranges may have a substantiallyconstant, high, transmission to viewers outside the narrow range ofangles for voltages across at least most of the range.

The circuitry may be adapted to operate in the first mode to use onlythe first one of the driving voltage ranges for each of the lateralregions to produce a substantially uniform arrangement of liquid crystalacross the panel.

The circuitry may be adapted to operate in the second mode to usedifferent respective ground electrode voltage arrangements to producethe at least two different arrangements of liquid crystal, the liquidcrystal arrangements being such as to have substantially the sametransmission to viewers within the narrow range of angles for differentrespective voltage arrangements and different respective transmissionsfor those voltages to viewers outside the narrow range of angles.

The circuitry may be adapted to cause the first and second arrangementsof liquid crystal to be skewed with respect to one another.

The circuitry may comprise a patterned electrode.

The narrow range of viewing angles may be disposed about the normal tothe display panel.

According to a second aspect of the present invention, there is provideda liquid crystal display panel for use in a display device fordisplaying an image by spatial light modulation, the display panel beingadapted to enable switching of liquid crystal in the panel betweenhaving a first configuration in a first mode to cause an image displayedusing the panel to be discernible from a wide range of viewing angles,and having a second configuration in a second mode to cause an imagedisplayed using the panel to be discernible substantially only fromwithin a narrow range of viewing angles.

According to a second aspect of the present invention, there is providedan operating program which, when loaded into an apparatus, causes theapparatus to become apparatus according to the first aspect of thepresent invention.

The operating program may be carried on a carrier medium. The carriermedium may be a transmission medium. The carrier medium may be a storagemedium.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings, in which:

FIG. 1 is a side view of a display panel according to a first embodimentof the present invention showing operation in wide and narrow viewingmodes;

FIG. 2 is a chart for use in explaining operation of the firstembodiment;

FIG. 3 is a block diagram illustrating a display device for use inexplaining a second embodiment of the present invention;

FIG. 4 is a plan view of a display panel for use in the secondembodiment, showing operation in wide and narrow viewing modes;

FIG. 5 is a flow diagram illustrating the operation according to a firstexample of the second embodiment;

FIGS. 6(A) to 6(C) is a schematic diagram illustrating the modificationof data values in the second embodiment;

FIG. 7(A) is a chart illustrating a linear data value-to-luminanceresponse in the second embodiment;

FIG. 7(B) is a chart illustrating a non-linear data value-to-luminanceresponse in the second embodiment;

FIG. 8 illustrates the scaling of the original and the masking images inthe first example of the second embodiment;

FIG. 9 illustrates the combining of the scaled original and the maskingimages in the first example of the second embodiment;

FIG. 10 illustrates the scaling of a masking image in a second exampleof the second embodiment;

FIGS. 11(A) to 11(C) illustrate various data value modificationarrangements suitable for use in the second embodiment;

FIG. 12 illustrates a possible effect of a sharp change in masking imagein the second embodiment;

FIG. 13 shows plan and side views of a display panel according to athird embodiment of the present invention showing operation in wide andnarrow viewing modes respectively;

FIGS. 14(A) and 14(B) are schematic illustrations of greyscale responseson and off axis respectively for a display panel according to a fourthembodiment of the present invention;

FIG. 15 illustrates a display panel suitable for use in the fourthembodiment;

FIG. 16 is a graph showing greyscale responses on-axis and off-axis forthe display panel of FIG. 15; and

FIGS. 17(A) and 17(B) show the viewing angle dependency respectively fortwo voltage ranges used in the fourth embodiment;

FIGS. 18(A) and 18(B) illustrate a fifth embodiment of the presentinvention;

FIGS. 19(A) and 19(B) illustrate a sixth embodiment of the presentinvention;

FIG. 20 illustrates the use of supplementary electrodes in the sixthembodiment;

FIG. 21 illustrates a seventh embodiment of the present invention;

FIG. 22 illustrates an eighth embodiment of the present invention;

FIG. 23(A) to 23(C) illustrates operation of the eighth embodiment;

FIG. 24 illustrates a staggered arrangement of patterned electrodes inthe eighth embodiment; and

FIG. 25 illustrates the difference in luminance seen off axis in theeighth embodiment.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a display device incorporating a liquid crystaldisplay panel 100 according to a first embodiment of the presentinvention. The liquid crystal display panel 100 displays an image byspatial light modulation, using opposed electrodes 101 and 103 disposedacross a layer of liquid crystal material 105. The electrode 103 issegmented so as to enable switching of the liquid crystal into two ormore different orientations within a single picture element. Within asingle picture element the same voltage is applied to all areas of thesegmented electrode 103. Preferably several regions of the two or moredifferent orientations are formed within a single picture element.Switching into the different orientations is controlled by the fringingelectric fields produced at the edges of the segmented electrode 103.Alternatively other methods of producing fringing electric fields, suchas protrusions on the electrode surface, may be used.

As explained in more detail below, the liquid crystal display panel 100also comprises circuitry 107 in the form of in-plane electrodes, forswitching the liquid crystal 105 between having a first configuration C1in a first (public or wide) mode and having a second configuration C2 ina second (private or narrow) mode. The first liquid crystalconfiguration C1 causes an image displayed using the panel 100 to bediscernible by a viewer from a wide range of viewing angles, while thesecond liquid crystal configuration C2 causes an image displayed usingthe panel 100 to be discernible by a viewer substantially only fromwithin a narrow range of viewing angles. As illustrated in FIG. 1, thedisplay device according to the first embodiment comprises only a singleliquid crystal display panel 100. No additional optical components orlayers are required in the first embodiment to achieve switching betweenthe two modes of operation.

FIG. 1 shows, in the left-hand portion, one suitable example of thefirst liquid crystal configuration C1 in the first (wide) mode ofoperation. The first liquid crystal configuration C1 in this example hastwo or more regions or domains (referred to from herein onwards asregions) of different liquid crystal orientation within a single pixel.Illustrated are four different regions R1 to R4, with regions R1 and R3having a first liquid crystal arrangement and regions R2 and R4 having asecond liquid crystal arrangement, different to the first liquid crystalarrangement. In FIG. 1 the combination of the four different regions R1to R4 forms a single picture element.

The first arrangement comprises liquid crystal having a substantiallyuniform first orientation of liquid crystal, while the secondarrangement comprises liquid crystal having a substantially uniformsecond orientation of liquid crystal, different to the firstorientation, with first and second orientations arranged in regionpairs. The first and second orientations are disposed substantiallysymmetrically about the normal to the display panel 100. In the exampleillustrated in FIG. 1, the first liquid configuration is a two-domainVertically Aligned Nematic (VAN) configuration.

In the first (wide) mode, the size of these regions R1 to R4 is smallerthan can be resolved by a viewer, for example of the order of 10 to 25μm across. The properties of the unresolvable regions average to producea wide viewing angle characteristic for the liquid crystal display panel100. This will be explained further with reference to FIG. 2 below. Themultiple regions of different liquid crystal orientation within a pixelare formed by fringing fields from the patterned electrode 103 or fromprotrusions on the electrode surface. An image is displayed on theliquid crystal display panel 100 in a known way by applying a switchingvoltage V1 across the liquid crystal layer 105.

In the second (narrow) mode, the in-plane electrodes 107 are used toprovide electric fields substantially within the plane of the liquidcrystal layer 105. To achieve this, a voltage of V2 is applied acrossadjacent in-plane electrodes 107, where V2 would typically be less thanV1. These in-plane fields overcome the effect of the fringing fieldsfrom the segmented electrode 103 and switch the liquid crystal layerinto a second liquid crystal configuration C2 having larger regions R5and R6, as illustrated in the right-hand portion of FIG. 1. Regions R5and R6 have different respective liquid crystal arrangements, the twoliquid crystal arrangements having substantially uniform differentrespective orientation of liquid crystal. The two different orientationsare disposed substantially symmetrically about the normal to the displaypanel 100.

The regions R5 and R6 in the second (narrow) mode are large enough to beresolved by a viewer, and would typically be much larger than a pixel,for example of the order of 1 mm or more across. As a result, and asexplained in more detail below, the effect of the regions R5 and R6 isclearly visible by an off-axis viewer (but the effect of the regions ishidden for an on-axis viewer). The regions R5 and R6 in the second(narrow) mode are arranged to provide a pattern that will obscure andconfuse the underlying image for an off-axis viewer in the second(narrow) mode. One example of such a pattern is a chequerboard, althoughany suitable pattern can be used.

The function of the first embodiment is further illustrated in FIG. 2.Plot 1 of FIG. 2 shows the transmission as a function of angle of viewfor a single VAN region, measured relative to the perpendicular to thepanel, i.e. for a single liquid crystal orientation. The angulartransmission function is asymmetric relative to the on-axis directiondue to the off-axis orientation of the liquid crystal molecules.

Suppose Plot 1 represents the angular transmission function for regionshave the liquid crystal orientation as shown in region R1, then theangular transmission function for regions having the liquid crystalorientation as shown in region R2 will be as for Plot 1, but reflectedin the vertical axis.

As explained above, in the first (wide) mode, neighbouring regions, forexample regions R1 and R2 in FIG. 1, are not resolvable by a viewer.Since these two regions R1 and R2 are not resolvable, the viewer sees anaverage transmission for the two regions R1 and R2 for all viewingangles. The resulting angular transmission function is shown in Plot 2of FIG. 2, which varies across viewing angle within acceptable bounds.

As explained above, in the second (narrow) mode the size of the VANregions R5 and R6 is such that, instead of an average of the twodomains, a viewer sees the domains as a distinct pattern because of thedifference in transmission between the two regions for a particularviewing angle. This difference will increase with viewing angle.

Due to the symmetry of liquid crystal orientation about theperpendicular to the display panel 100, when viewed on-axis both regionsR5 and R6 exhibit the same transmission, and so the effect of thedifferent regions is not apparent to the on-axis viewer. An image isdisplayed in the standard way by applying a switching voltage V1 acrossthe liquid crystal layer 105.

Away from the perpendicular direction, the two regions R5 and R6 givedifferent transmission for the same applied voltage. Therefore anoff-axis viewer will see the pattern of the VAN domains as a pattern ofdiffering brightness. This pattern will obscure the underlying image.The contrast between the two regions in the second (private) mode isillustrated by Plot 3 of FIG. 2, which is the difference in transmissionbetween adjacent regions R5 and R6 for each viewing angle.

In this way a good privacy function can be achieved with the firstembodiment of the present invention by switching the liquid crystallayer 105 itself, without the need for additional layers.

Instead of or as well as using electric field switching of the liquidcrystal layer 105 as described above, the alignment layer can also beswitched to produce the wide and narrow viewing modes. One method ofswitching the alignment is the use of very fine patterning of analignment layer as described by Kim et al., ‘Surface alignmentbistability of nematic liquid crystals by orientationally frustratedsurface patterns’, Applied Physics Letters, Vol 78, Is 20 (2001) 3055.Another method, disclosed in EP0856164 and Kitson and Geisow,‘Controllable alignment of nematic liquid crystals around microscopicposts: Stabilization of multiple states’ Applied Physics Letters, Vol80, Is 19 (2002) 3635, uses an alignment layer consisting of periodicmicrostructures. These microstructures also induce bistable ormultistable alignment of the liquid crystal. A further method, disclosedin U.S. Pat. No. 6,549,255, uses a polymer alignment layer whosealignment properties can be switched with an applied field.

A second embodiment of the present invention, also making use of anin-panel liquid crystal switching technique to achieve switching betweena public and private mode, will now be described with reference to FIGS.3 to 12.

FIG. 3 is a block diagram illustrating an image display system 1 for usein explaining a second embodiment of the present invention. The imagedisplay system 1 comprises an image processor 10, a display controller20 and a display device 30. The display device comprises a display panel32 and a non-linear component 34. A liquid crystal display panel 200according to the second embodiment is illustrated in FIG. 4, and isintended to replace the display panel 32 and a non-linear component 34of FIG. 3, as will be described below. Referring to FIG. 3, an originalimage I is to be displayed on the display panel 32 of the display device30. The original image I is represented by a plurality of imageelements, which may correspond to pixels of the display panel 32 orsub-pixels of the display panel 32. If the original image I is displayeddirectly on the display device 30, then the image will be viewable bothby a viewer positioned on-axis at a position P1, and by a viewerpositioned off-axis at a position P2. The first and second positions P1and P2 are within first and second viewing regions R1 and R2respectively.

The second embodiment of the present invention enables a mode ofoperation in which the image seen by a first viewer located at positionP1 relative to the display device 30 is substantially the same as theoriginal image I, whereas the image seen by a second viewer located atposition P2 relative to the display device 30 is different from theoriginal image I. The masking image M is used for this purpose, as willnow be explained in connection with a first example of the secondembodiment.

Operation of the first example of the second embodiment will bedescribed with reference to FIGS. 5 to 9. FIG. 5 is a flow diagramproviding an overview of the operations performed by the image processor10 in the first example of the second embodiment. Before considering indetail the steps performed by the image processor 10, a more generaldescription of the concept underlying an embodiment of the presentinvention will first be described with reference to FIGS. 6 and 7.

When a viewer is located more than a predetermined distance away fromthe display panel 32 of the display device 30, the viewer is unable toresolve each individual pixel or image element being displayed. Aguideline for use in estimating an adequate viewer-display separationfor this to be the case is provided in “Color and Light in Nature”, D.Lynch & W Livingston, Cambridge University Press, 1995, which suggeststhat the eye's resolution is limited to 1 arc minute. Applying this tothe present embodiment, two pixels or image element should preferablysubtend an angle of less than 1 arc minute. It is to be understood thatthis is merely a guideline and that other resolutions may be applicablein different circumstances.

Instead of being able to resolve each individual pixel or image element,the human eye will spatially average a localised group of displayedimage elements to perceive a single overall luminance. It should benoted that the localised group of displayed image elements may notcorrespond to arrangement of these image elements in the original image,for example because the image elements may be interlaced or otherwisere-arranged before display. The second embodiment of the presentinvention takes advantage of this, together with the datavalue-to-luminance response of the display device 30, as will now beexplained.

FIG. 6(A) shows one such localised group comprising two image elementshaving the same data value. Before display, the image processor 10 in anembodiment of the present invention splits the original data valueequally into two new data values, such that the data value of one of theimage elements is equal to the original data value minus the splittingamount, and the data value of the other image element is equal to theoriginal data value plus the splitting amount.

When displayed on a display device having a linear datavalue-to-luminance response, a viewer will perceive the two modifieddata values of the localised group to have the same overall luminance asthose image elements would have done without such modification. This isbecause, due to the linear response of the display device, the singleoriginal data value maps to the same luminance as the average luminanceof the two modified image elements. This is illustrated in FIGS. 6(B)and 7(A).

On the other hand, when the modified image elements in the localisedgroup are displayed on a display device having a non-linear datavalue-to-luminance response to a viewer in a predetermined positionrelative to the display device, the luminance of these image elements isno longer spatially averaged by the eye of the viewer to have the sameoverall luminance as those image elements would have done without suchmodification. Instead, the viewer perceives a luminance which differsfrom a straight average by an amount which depends on the non-linearityof the display. This is illustrated in FIG. 7(B) and FIG. 6(C).

The switching voltage applied to a pixel in a Liquid Crystal Display(LCD) device is usually compensated such that, when viewed on-axis, achange in the data sent to the pixel causes a proportional change in theobserved luminance. However, light passing through the panel 32 at anangle other than the normal to the panel 32 will travel a differentoptical path length through the Liquid Crystal (LC) and will thereforebe affected differently. This change in optical path length canintroduce a non-linear relationship between the pixel data and theobserved luminance off-axis.

Because of this, the data value-to-luminance response to a viewer in afirst position P1 substantially normal to the display panel 32 will besubstantially linear, and the image perceived by such a viewer throughspatial averaging will be substantially the same as the original image.

On the other hand, the data value-to-luminance response to a secondviewer at position P2 off-axis relative to the display panel 32 can benon-linear, and the image perceived by such a viewer through spatialaveraging can therefore be different to the original image.

The required difference in the first and second data value-to-luminanceresponses is sometimes inherent in the properties of the display deviceitself. However, some displays are compensated to remove thenon-linearity of the LC, such that the on-axis and off-axis responsesare both substantially linear. For such devices, the off-axisnon-linearity can be reintroduced by the introduction of the non-linearcomponent 34 shown in FIG. 3 to modify the viewing characteristics ofthe display. This non-linear component 34 can be a simple unpatterned LClayer. The non-linear component 34 can also be switchable, so that itneed only be activated when required. It will be appreciated, however,that for display devices inherently having different luminance responseson- and off-axis the non-linear component 34 is not necessary, althoughthe use of such a component may enhance the performance.

Alternatively, and this is the approach taken in the second embodimentof the present invention, the LC panel itself may be operable to switchbetween two modes of operation, one in which the display has a constantluminance response and another in which the display has a non-linearluminance response for viewer P2. This would achieve the same effect,but without the need for the extra non-linear component 34. The in-panelswitching technique of the second embodiment of the present invention isintended to ensure a non-linear data value-to-luminance responseoff-axis for use with the method disclosed in the above-mentionedco-pending application. The display panel used for second embodimentwill now be described in more detail with reference to FIG. 4.

FIG. 4 illustrates a single-layer liquid crystal display panel 200according to the second embodiment. The display panel 200 is intended toperform the function of the display panel 32 and the non-linearcomponent 34 combined. The liquid crystal in the display panel 200 isoperated to switch between a first configuration C1 in the first (wide)mode of operation as illustrated in the top portion of FIG. 4, and asecond configuration C2 in the second (narrow) mode of operation asillustrated in the bottom portion of FIG. 4.

In the illustration of FIG. 4, the first liquid crystal configuration C1is a four-domain (four-region) twisted nematic (TN) configuration, withfour regions R11 to R14 shown. Each of the four regions R11 to R14 hasdifferent TN arrangements, each arrangement oriented at 90 degrees withrespect to another arrangement.

The second liquid crystal configuration C2 is a two-domain (two-region)twisted nematic (TN) configuration, with two regions R15 and R16 shown.Region R15 occupies the same space as regions R11 and R12, while regionR16 occupies the same space as regions R13 and R14. Region R15 has a TNarrangement that is oriented 180 degrees with respect to the TNarrangement of region R16.

The four-region liquid crystal configuration C1 can be fabricated bypatterning of the liquid crystal alignment, for example by multi-rubbingor photoalignment. In the first (wide) mode the average transmission ofthe four regions provides a linear data value-to-luminance response forlight transmitted at oblique angles. In this mode, light passes throughall four regions having different arrangements of liquid crystal. Foreach individual region there will be certain directions for which thetransmission of light travelling at oblique angles will have a lineardata value-to-luminance response and other directions for which thetransmission of light travelling at oblique angles will have anon-linear data value-to-luminance response. However due to theaveraging of the transmitted light over the four regions the overalldata value-to-luminance response is substantially linear in alldirections.

In the second (narrow) mode, the alignment is switched to a two-regionTN configuration C2, with the regions being orientated so that in thehorizontal viewing plane (a plane normal to the page and parallel withthe top and bottom edges of the page) the data value-to-luminanceresponse is non-linear for light transmitted at oblique angles. In thismode, both regions R15 and R16 are arranged so that the directions forwhich the transmission of light travelling at oblique angles will have anon-linear data value-to-luminance response are in the horizontalviewing plane. Therefore the overall data value-to-luminance response,which is an average of the response of the two regions, is non-linear.When such a mode is used in combination with an image processing methoddisclosed in the above-mentioned co-pending application, a good privacyfunction can be achieved.

One method of switching the alignment is the use of very fine patterningof an alignment layer as described by Kim et al., ‘Surface alignmentbistability of nematic liquid crystals by orientationally frustratedsurface patterns’, Applied Physics Letters, Vol 78, Is 20 (2001) 3055.Another method, disclosed in EP0856164 and Kitson and Geisow,‘Controllable alignment of nematic liquid crystals around microscopicposts: Stabilization of multiple states’ Applied Physics Letters, Vol80, Is 19 (2002) 3635, uses an alignment layer consisting of periodicmicrostructures. These microstructures also induce bistable ormultistable alignment of the liquid crystal. A further method, disclosedin U.S. Pat. No. 6,549,255, uses a polymer alignment layer whosealignment properties can be switched with an applied field.

Instead of or as well as switching the alignment layer as described inthe embodiment above, the liquid crystal layer can also be switchedusing an electric field to produce the wide and narrow viewing modes.Examples are applying an electric field across the liquid crystal layer,applying an electric field in the plane of the liquid crystal layer, andapplying fringe fields from a patterned electrode.

A more detailed description of the operation of the first example of thesecond embodiment will now be provided with reference to FIGS. 5, 8 and9.

The image elements in the original image I would usually take any valuein the range from 0 to 255. Because of this, the splitting as describedabove with reference to FIG. 6 of a data value close to the minimum ormaximum data value would potentially result in a modified data valuefalling outside the normal range of data values allowed. In order toprevent this, in step S1 of the first example of the second embodiment,the original image I is scaled and centred so as to have a new,compressed data value range; this is illustrated in the top half of FIG.8. In step S2, the data value range of the masking image M is scaledsuch that the minimum data value is 0 and the maximum data value isequal to the minimum of the scaled original image; this is illustratedin the bottom half of FIG. 8.

FIG. 9 shows example cross-sections through the scaled original image Iand the scaled masking image M. The data value of the masking image atany point determines the level of splitting to be used for the imageelement at a corresponding position of the scaled original image I. Thelevel of splitting is proportional to the scaled data value of themasking image M, with neighbouring image elements of the scaled originalimage I being increased and decreased respectively by the splittinglevel determined from the scaled masking image M. The right-hand part ofFIG. 9 shows the result of the combination of the scaled original andmasking images I and M, with the greatest degree of splitting occurringat the positions of the maxima of the masking image M.

In the first example of the second embodiment, the splitting amounts arerespectively added and subtracted from the scaled original image I byfirst inverting half of the scaled masking image M data values in stepS3, and then adding the resulting pattern of data values to the scaledoriginal image I in step S4. The resulting image is then viewed in S5 bythe viewer. The on-axis viewer will perceive an image through spatialaveraging that is substantially the same as the scaled original image,while the off-axis viewer positioned at P2 will perceive an image thatis different to the original image, resembling at least to some extentthe masking image M. For a good privacy mode, the masking image would besuch as to provide a high degree of visually confusing information tothe off-axis viewer.

A second example of the second embodiment of the present invention willnow be described with reference to FIG. 10. The second example of thesecond embodiment is similar to the first example of the secondembodiment, using the apparatus of FIG. 3, and accordingly will onlybriefly be described here. The difference between the first and secondexamples of the second embodiment results from the manner in which theoriginal and masking images I and M are scaled and combined. In thefirst example of the second embodiment, the scaling of the originalimage I into a narrower range of data values results in a decrease inthe contrast of the image, whereas the method that is performed in thesecond example of the second embodiment is such that the contrast of theoriginal image is not sacrificed.

In the second example of the second embodiment, the degree of splittingis partially determined by how far the data value of a particular imageelement is from the closest edge of the allowed data value range. Imageelements having data values in the middle of the range will be split themost, while image elements near 0 or 255 will be split the least. Imageelements at either extreme of the range will not be split at all. Thisensures that the modified data value does not fall outside the allowedrange of data values.

Therefore, in the second example of the second embodiment, thedifference between the original data value and the maximum or minimumdata value, whichever is closer, is calculated, and this effectivelysets the maximum level of splitting. This is illustrated in the top halfof FIG. 10.

The masking image M is then scaled according to the maximum level ofsplitting allowed for each image element to produce the scaled maskingimage M to be combined with the original image I. The actual combinationcan be performed in a similar way as described above with reference tothe first example of the second embodiment.

Therefore, the overall process in the second example of the secondembodiment is similar to the overall process in the first example of thesecond embodiment. Referring to FIG. 5, in the second example of thesecond embodiment step S1 is omitted, while the scaling of the maskingimage M in step S2 is performed as described above with reference toFIG. 10. Compared to the first example of the second embodiment, thesecond example of the second embodiment produces a strong effect onimages having a pictorial content, but since saturated pixels undergolittle or no splitting, the second example of the second embodiment willhave no effect on pure black and white text.

Image elements having corresponding modifications performed on them, forexample splitting upwards or splitting downwards, can be arranged tocorrespond to lines of the display device 30, as shown in FIG. 11(A).Alternatively, corresponding image elements can be arranged in columns,or in a chequerboard pattern as shown in FIG. 11(B). With thearrangements shown in FIGS. 11(A) and 11(B), each image elementcomprises three separate RGB colour components with each colourcomponent being represented by a data value. Each data value of theimage element is modified in the same way.

Alternatively, where each image element comprises three separate RGBcolour components, each colour component may be treated independently,so that different colour component data values for the same imageelement may be split differently. One possible arrangement is shown inFIG. 11(C).

Other ways of combining the original and masking images I and M arepossible. For example, as with the first example of the secondembodiment, the contrast of the original image I could be reduced, butan asymmetric compression and splitting could be used rather thancentring the compressed original image I. This would mean that thecontrast of either the light or dark areas are retained preferentially,with the masking image M being compressed as in the first example of thesecond embodiment but also scaled according to the original image suchthat the modified data values do not fall outside the allowed range ofdata values. Other methods would be readily apparent to the skilledperson, and any combination of the above methods can be used.

Although it is generally assumed above that the on-axis luminanceresponse of the display device 30 is linear, such that equal splittingof data values will result in the viewer seeing substantially theoriginal image through spatial averaging, if in fact the on-axisluminance response is non-linear, then the splitting can easily becompensated to account for this so that the on-axis viewer still seessubstantially the original image through spatial averaging.

Any type of masking image M can be used, depending on the intendedapplication. For example, the masking image could comprise a colour orblack and white chequerboard pattern or random noise for apublic/private mode application. The masking image could also comprise alogo or other image, text or any other form of information for displayonly on particular types of display device or to viewers located inparticular positions. Animated masking images can also be used.

The image elements in a group, for example the two image elements shownin FIGS. 6(A) to (C), can be averaged before adding and subtracting themasking image, or it can be assumed that they take the same value.

For a masking image, that is not aligned in any way with the image beingdisplayed, it could be the case that the image elements in any localisedgroup would be split by different amounts according to the correspondingdata values of the masking image M. It would also be possible to averagethe masking image data values such that the same degree of splitting isapplied to both or all image elements in a group. The number of imageelements averaged together can be two or more. Although the exampledescribed above with reference to FIG. 6 showed the image elements beingconsidered in pairs, this is not essential, and any number of imageelements can be considered in a localised group of image elements, itmerely being necessary that the localised group of image elements areperceived by the on-axis viewer through spatial averaging to havesubstantially the same overall luminance as those image elements wouldhave done without modification. The spatially averaged pixels could alsobe reversed time sequentially, so that in one frame a pixel may have themasking image added, and in a subsequent time frame the masking imagewould instead be subtracted (with an equivalent reversal for itsneighbour).

Although the above description of the second embodiment has referred tolocalised groups of displayed image elements, it is to be understoodthat this is a useful concept to adopt to ensure proper spatialaveraging is provided, but does not necessarily imply the processing ofimage elements in separate, individual groups. The image elements can beprocessed without reference to any grouping, but with an appropriateglobal pattern of modification to ensure correct local spatial averaging(for example, as shown in FIGS. 11(A) to 11(C)).

The masking image should preferably have smoothly-varying data values,since any abrupt changes may have a minor effect on the perceived imageon-axis in certain circumstances. FIG. 12 illustrates why an abruptchange in the masking image could leave an artifact in the perceivedimage. This can be counteracted with suitable modification to a methodused in the second embodiment, for example a pre-processing stage toalter the masking image to avoid or reduce such an effect, or a changeto the algorithm used to combine the masking image to prevent or reducesuch an effect.

In summary, a display device according to the second embodimentcomprises a liquid crystal display panel for displaying an image byspatial light modulation, and circuitry for switching liquid crystal inthe panel between having a first configuration in a first (public) modeto cause an image displayed using the panel to be discernible from awide range of viewing angles, and having a second configuration in asecond (private) mode to cause an image displayed using the panel to bediscernible substantially only from within a narrow range of viewingangles. The second configuration of liquid crystal is such as to causean image-confusing pattern to be visible in the image discerned by aviewer outside the narrow range of angles. The liquid crystal in thefirst configuration comprises a plurality of lateral regions each havingone of at least two different arrangements of liquid crystal. The firstconfiguration regions are sized so as not to be resolvable by a viewer.The liquid crystal in the second configuration may comprise a pluralityof lateral regions each having one of at least two differentarrangements of liquid crystal or may comprise a single region. Theliquid crystal in the second configuration is arranged to provide thedisplay panel with a substantially linear data value-to-luminanceresponse for viewing angles within the narrow range and a substantiallynon-linear data value-to-luminance response for viewing angles outsidethe narrow range. The first configuration regions are adapted such thatlight travelling at an angle outside the narrow range of angles passesthrough at least two regions having different arrangements of liquidcrystal so as to have a substantially linear data value-to-luminanceresponse. The second configuration regions are adapted such that lighttravelling at an angle outside the narrow range of angles has asubstantially non-linear data value-to-luminance response.

As described above, in the first example of the second embodiment of thepresent invention, neighbouring data values are modified to produce animage that, when viewed through a linear display device (on-axis) willbe spatially averaged by the human eye back to the original image, butwhen viewed through a non-linear display device (off-axis) willintroduce a component of the degree of splitting used. If the degree ofsplitting is varied across the image by an amount proportional to asecond image, then when viewed off-axis the original and the secondimage will both be visible. The second image is the masking image Mdescribed above. If the masking M has a confusing patterning such as achequerboard or company logo, then the original image will besubstantially hidden to an off-axis viewer. This provides a private modeof operation, in which only the on-axis viewer has an undisturbed viewof the original image.

The second embodiment of the present invention provides anelectronically-switchable method for producing viewing anglerestriction. A custom masking image can be used, which may be a movingimage in order to provide enhanced confusion to unauthorised viewers.The second embodiment does not require shuttered glasses as do someknown techniques, and can be applied to the whole or part of thedisplay. Privacy can be produced in both horizontal and verticaldirections if there is non-linearity in both planes. The privacy leveland area can be dependent on the content being displayed, and variableview-angle restrictions can be provided by changing the degree ofsplitting used. An embodiment of the present invention provides a lowcost, switchable system for producing switching view-angle restriction.

The image processing parts of the second embodiment can be implementedin hardware, or in software, or a combination. An operating program forimplementing the second embodiment can be stored on a computer-readablemedium, although an operating program embodying the present inventionneed not be stored on a computer-readable medium and could, for example,be embodied in a signal such as a downloadable data signal provided froman Internet website. The appended claims are to be interpreted ascovering an operating program by itself, or as a record on a carrier, oras a signal, or in any other form.

It will be appreciated that the image processing technique of the secondembodiment can be used in combination with any of the other embodimentsdescribed herein to enhance the effectiveness of the private mode. Inthis respect, the display panels used in each of the embodimentsdescribed herein will, at least to some extent, exhibit the non-linearcharacteristics required for the image processing technique to provideat least some additional benefit in the private mode.

FIG. 13 illustrates a display device incorporating a liquid crystaldisplay panel 300 according to a third embodiment of the presentinvention. In the third embodiment the first (wide) mode of the liquidcrystal display is provided by switching a liquid crystal layer 305 withan electric field substantially parallel to the layer a using a firstset of (in-plane) electrodes 307, 308. This in-plane switching (IPS) isknown to give a wide viewing angle. The liquid crystal configuration C1in the first (wide) mode of operation is illustrated in the top portionof FIG. 13, which is a plan view of one liquid crystal in-planeswitching cell.

The second (narrow) mode of operation is provided by switching theliquid crystal layer 305 with an electric field applied across theliquid crystal layer (electrically controlled birefringent or ECBswitching) using a second set of electrodes 301, 303. The switching ofthe liquid crystal layer 305 out of the plane of the layer gives anarrow viewing angle. The liquid crystal configuration C2 in the second(narrow) mode of operation is illustrated in the bottom portion of FIG.13, which is a side view of one liquid crystal ECB switching cell.

Alternatively, the first (wide) and second (narrow) viewing modes can beprovided in an embodiment of the present invention by driving a liquidcrystal display panel in two different respective voltage ranges. Anexample of a suitable such device is shown in FIG. 15, with associatedviewing angle characteristics shown in FIG. 17.

A fourth embodiment will be described with reference to FIGS. 14(A) to17. In this embodiment, switchable privacy is achieved using an LC modewhich has two voltage ranges, denoted in FIGS. 14(A) and 14(B) as rangesA and B, with similar greyscale variation on axis but a single voltagerange, denoted as A in FIGS. 14(A) and 14(B), with normal greyscalevariation off axis. In the public mode, the voltage range A havingnormal greyscale variation both on axis (FIG. 14(A)) and off axis (FIG.14(B)) is used and good image quality is seen at all viewing angles. Inthe private mode, some of the pixels achieve the desired greyscale onaxis (FIG. 14(A)) using the first voltage range A and other pixelsachieve the same greyscale on axis (FIG. 14(A)) using the second voltagerange B. Pixels that use the first voltage range A will appear normalboth on and off axis (see pixels in the example image on the right-handside of FIGS. 14(A) and 14(B) respectively that are linked by an arrowto voltage range A). However, pixels that use the second voltage range Bwill not appear normal off axis (see pixels in the example image on theright-hand side of FIG. 14(B) that are linked by an arrow to voltagerange B). By patterning the pixels that use the first and second voltageranges A and B, a confusing image will appear off axis (see the exampleimage on the right-hand side of FIG. 14(B)).

An example of such a greyscale response is shown in FIGS. 14(A) and14(B). For each greylevel on axis (FIG. 14(A)), there are two voltagesthat can be used. However, these two voltages do not give the samegreylevels off axis (FIG. 14(B)). By patterning the pixels which use thefirst and second voltage ranges A and B a confusing image will be seenoff axis (FIG. 14(B)). The pattern is not seen on axis (FIG. 14(A))because the two voltage ranges A and B are matched to the samegreylevels for this viewing angle.

An example of a liquid crystal mode that can achieve this is shown inFIG. 15, and would be readily understood by the skilled person. FIG. 16shows that, for the first voltage range of ˜0.9V to 1.6V, the greyscalecurve on axis is similar to the greyscale curve off axis (+/−45 deg).However, for the second voltage range of ˜1.6V to 2.4V, the greyscalecurve is inverted but similar in range on axis but is virtuallyunchanged off axis. Therefore two voltages can be selected to giveidentical greylevels on axis but these voltages will give very differentgreylevels off axis. FIGS. 17(A) and 17(B) show the viewing angledependency of the greylevels for the two voltage ranges. Only the lowvoltage range is used for the public mode, while a pattern of low andhigh voltage ranges is used for the private mode; by matching greylevelson axis the pattern will only be seen off axis.

Therefore, in the fourth embodiment, two voltage ranges are used toachieve similar greylevels on axis, one of these voltage ranges givingnormal greyscale variation off axis and the other giving abnormalgreyscale off axis. Driving the panel only in voltage the first voltagerange gives good viewing at all angles. Driving the panel in the secondvoltage range gives abnormal viewing off axis. Privacy can be enhancedby patterning the pixels which use first and second voltage ranges suchthat the abnormal viewing off axis results in a pattern which confusesthe image seen by the off axis viewer.

A fifth embodiment of the present invention will now be described withreference to FIGS. 18(A) and 18(B). In the fifth embodiment, the numberor characteristics of the liquid crystal orientations in the secondconfiguration (private mode) are modified with respect to the firstconfiguration (public mode), such that the image displayed in the secondconfiguration (private mode) is visible over a narrower range of viewingangles than in the first configuration (public mode). The switching ofthe number or orientation of the liquid crystal regions is achieved byuse of an in-plane electric field.

The liquid crystal mode employed is the Continuous Pinwheel Aligned(CPA) mode. In this mode, the liquid crystal molecules are verticallyaligned when no voltage is applied. When a substantially uniformelectric field is applied, the presence of a ‘rivet’ protrusion on theelectrode surface, along with the fringing fields at the edge of thepixel, results in pairs of liquid crystal orientations tiltedsymmetrically on opposite sides of the protrusion. These pairs oforientations are arranged in a continuous ‘pinwheel’ structure whenviewed from the normal to the liquid crystal layer. The average of thecontinuous pinwheel liquid crystal orientations provides an inherentlywide viewing angle in all azimuthal directions. This first configurationis shown in FIG. 18(A). The electrode is segmented, as described below,but substantially the same voltage is applied to each segment of theelectrode to achieve a substantially uniform electric field.

A second (narrow) viewing angle configuration is provided by patterningthe electrode and applying different voltages to different electrodesegments, as shown in FIG. 18(B). The patterned electrode producesadditional electric fields substantially within the plane of the liquidcrystal layer. These in-plane fields overcome the effect of theprotrusion on the liquid crystal orientations and reduce the number oforientations; in the example depicted in FIG. 18(B) the number oforientations is reduced to two. The average of the resulting two liquidcrystal orientations will have a wide viewing angle for certainazimuthal directions and a narrower viewing angle for other azimuthaldirections.

Therefore, in the first (wide) viewing angle configuration the patternedelectrodes are set to substantially the same voltage within a givenpixel, and the protrusion produces continuous pinwheel liquid crystalorientations. In the second (narrow) viewing angle configuration, avoltage is applied between adjacent in-plane electrodes to modify thenumber or characteristics of the liquid crystal orientations.

The orientation of the in-plane electrode segments can be arranged inregions on a scale large enough to be visible to a viewer. For aparticular azimuthal direction, some of these regions will have wideviewing angle and some a narrow viewing angle. Therefore an off-axisviewer will see the different regions as a pattern of differentbrightness. This image confusion pattern will obscure the underlyingimage.

It is advantageous if the patterned in-plane electrode segments form theground electrode for the pixel, rather than the thin film transistor(TFT) electrode. In this case, an incremental voltage negative ΔV can beapplied to half of the in-plane electrodes and positive ΔV can beapplied to the other half. This incremental voltage generates anin-plane field, but leaves the average voltage of the electrode on oneside of the liquid crystal layer at zero volts (ground). A singleincremental voltage ΔV can be applied to all the in-plane electrodesacross the whole panel to switch from the first (wide) viewing angleconfiguration to the second (narrow) viewing angle configuration. Noextra TFTs are required at the pixels to switch between configurations.

The pitch of the in-plane electrode segments required to modify thenumber or characteristics of the liquid crystal orientations will varyaccording to the liquid crystal mode. The pitch can be varied from beingsubstantially equal to the thickness of the liquid crystal layer to apitch such that there are only two in-plane electrode segments perpixel. In the case of two in-plane electrode segments per pixel, thesemay be located at the edges of the pixel, and may be located outside thelight-transmitting aperture of the pixel. Generally, the stronger thetendency to form the first liquid crystal configuration the greater thenumber of electrode segments per pixel would be required to switchadequately to the second liquid crystal configuration.

A sixth embodiment of the present invention will now be described withreference to FIGS. 19(A), 19(B) and 20. In this embodiment, an alignmentlayer consisting of periodic microstructures is used to enable switchingbetween a first configuration having a wide range of viewing angles anda second configuration having a narrow range of viewing angles.

One type of grating aligned nematic liquid crystal cells have a surfacewhich, due to periodic micro-structures, is switchable between acontinuous liquid crystal director structure, which is approximatelyhomeotropic, and a “defect” structure, which is approximately planaraligned. These states are bistable and switching between the two isachieved by a DC field pulse coupling flexoelectrically to the splay andbend regions of the director field. Such a surface used in a continuouspinwheel aligned (CPA) liquid crystal allows switching in thisembodiment between the usual homeotropic aligned VAN structure, in whichan applied high frequency field causes a radial director distributionabout the centre of the pixel, and a hybrid aligned nematic (HAN) likestructure in which this radial distribution is disrupted by thepreferential alignment direction caused by the grating. These are shownrespectively in FIGS. 19(A) and 19(B).

As the switching from continuous to defect state requires a DC pulse,the usual AC applied field used in an ASV pixel will still be useable toprovide greylevel control.

The radial director in the continuous state provides an inherently wideviewing angle to all azimuths, whereas the two “HAN like” liquid crystalorientations will have a wide viewing angle for certain azimuthaldirections and a narrower viewing angle for other azimuthal directions.The orientation of the periodic micro-structures can be patterned inregions on a scale large enough to be visible to a viewer. For aparticular azimuthal direction some of these regions will have wideviewing angle and some a narrow viewing angle. Therefore an off-axisviewer will see the different regions as a pattern of differentbrightness. This image confusion pattern will obscure the underlyingimage.

The fringing fields at the edge of the pixel, which, along with the“rivet” protrusion in the centre, promote the radial directordistribution, will still be in effect in the defect state. This mayhinder the switch to the more linear director alignment in the defectstate. For this reason, the electrode area on the lower substrate of thecell could be expanded beyond the light-transmitting aperture of thepixel to remove these fringe field effects from the visible region. Thiscould be done by having supplementary electrode regions which would onlybe switched on when the display was in the defect state to assistrestriction of the viewing angle; this is shown in FIG. 20.

A seventh embodiment of the present invention will now be described withreference to FIG. 21. In the seventh embodiment, the display is composedof pixels that are each subdivided into a part having a narrow viewingangle LC mode and a part having a wide viewing angle LC mode. The twodifferent LC mode parts may be arranged in separate columns (as shown inFIG. 21) or patterned in some other way (a chequerboard pattern, forexample). One of the LC modes has an intrinsic wide viewing anglecharacteristic and the other mode has an intrinsic narrow viewing anglecharacteristic. An example of a wide viewing angle mode is the in-planeswitching (IPS) mode and an example of a narrow viewing angle mode isthe electrically controlled birefringence (ECB) mode, so that theseventh embodiment can be considered to be similar to the thirdembodiment described above (and accordingly only the main differencesbetween the third and seventh embodiments are described here). Where thewide and narrow LC modes are arranged in alternating columns as shown inFIG. 21, for each pixel there is a wide and narrow viewing anglesub-pixel.

In the public mode, the display panel may be operated so that only thepixels having a wide viewing angle LC mode operate. Alternatively, boththe wide and narrow viewing mode pixels may operate simultaneously, sothe on-axis user receives the advantage of an image with twice thebrightness or spatial resolution. In the private mode, the display panelmay be operated so that only the pixels having a narrow viewing angle LCmode operate. Alternatively, a privacy pattern can be realised byoperating the narrow viewing angle LC mode pixels in some regions andthe wide viewing angle LC mode pixels in other regions. An off-axisviewer will see the different regions as a pattern of differentbrightness. This image confusion pattern will obscure the underlyingimage.

An eighth embodiment of the present invention will now be described withreference to FIGS. 22 to 25. The eighth embodiment, similar to thefourth embodiment described above, employs two different voltage rangesto achieve similar greylevels on-axis but differing greylevels off axis.

For a continuous pinwheel alignment (CPA) mode liquid crystal display(LCD), the radially distributed director structure is induced by both aprotruding “rivet” on the upper cell surface in the centre of the pixel,and fringing fields from the edge of the square electrode area on thelower substrate. The lower substrate contains the thin film transistor(TFT) array, and the upper substrate has the common ground electrode.

In the eighth embodiment, this common ground electrode is divided intotwo interdigitated electrodes, each covering one side of every sub-pixelpinwheel domain, as is illustrated in FIG. 22 in plan view. An equal butopposite bias voltage is applied to each side of the sub pixel, i.e. thedriving voltage applied to every pixel to display an image is altered by+ΔV for one half of the sub-pixel and −ΔV for the other half. Anadvantage of patterning the ground electrode in this way is that asingle bias voltage ΔV can be applied to all the electrodes across thewhole panel. Also, no extra TFTs are required at the pixels.

Due to the largely linear greylevel (voltage-luminance) curve observedfrom a CPA pixel on-axis as shown in FIG. 23(A), the respectiveincreases and decreases in drive voltage produce a luminance whichaverages out substantially to the original luminance that would beobserved with a common ground electrode at 0V, so that very littlechange occurs and the perceived image remains unaltered.

To the off-axis viewer, however, the resulting luminance depends greatlyon whether the viewer is positioned to the side of the subpixel whichhas the increased electric field, or to the side of the subpixel havingthe decreased electric field. This asymmetry in the voltage response isillustrated in FIGS. 23(B) and 23 (C), and is due to the liquid crystaldirector tilting towards or away from the viewer.

The applied bias will therefore cause the display to appear brighterfrom one side than the other. If the interdigitated electrodes arestaggered, so that some portions of the display are made to appearbrighter to the left side and darker to the right, and the remainingregions have the opposite effect, an image confusion pattern can begenerated for viewers on either side, and the information on the displayis concealed to anyone but the on-axis viewer. One possible staggeringarrangement for the interdigitated counter electrode is shown in FIG.24.

The difference in luminance between a sub-pixel biased to the left and asub-pixel biased to the right, for a viewer one side of the display,determines the strength of this confusion pattern. This has beencalculated for a CPA type pixel between achromatic circular polarisers,as used in mobile phone LCDs, for a range of +/− bias voltages, and hasbeen found to have the dependence on the initial (unbiased) greylevelvoltage shown in FIG. 25.

This shows that the best privacy would be obtained for pixel display amid-brightness image.

It will be appreciated that each of the above-described embodiments isnot limited to operation using the particular liquid crystalconfigurations described. Each embodiment of the present invention canbe used with any of the following types of liquid crystal display: twoor four domain Twisted Nematic (TN) or Super Twisted Nematic (STN); twoor four domain Vertically Aligned Nematic (VAN) and Twisted VerticallyAligned Nematic (TVAN); two or four domain Hybrid Aligned Nematic (HAN);Multidomain Vertical Aligned (MVA); and Continuous Pinwheel Aligned(CPA). In addition liquid crystal modes that have intrinsic bistability,such as Bistable Twisted Nematic (BTN), Bistable Hybrid Aligned Nematic(BHAN), Zenthally Bistable Nematic (ZBN) and azimuthally bistable modesproduced by gratings or other surface structures can be employed. Othersuitable liquid crystal modes would be readily available to thoseskilled in the art.

An embodiment of the present invention can be applied to any type ofdisplay device, for example those on mobile phones, Personal DigitalAssistants (PDA), Electronic Point of Sale (EPoS) kiosks, laptopcomputers or desktop monitors.

1. A display device comprising a liquid crystal display panel fordisplaying an image by spatial light modulation, and circuitry forswitching liquid crystal in the panel between having a firstconfiguration in a first mode to cause an image displayed using thepanel to be discernible from a wide range of viewing angles, and havinga second configuration in a second mode to cause an image displayedusing the panel to be discernible substantially only from within anarrow range of viewing angles, wherein an image is represented by aplurality of image elements, and the display device further comprisesmeans for modifying the respective data values of at least some of theimage elements such that when the modified image is displayed in a firstscenario using a display panel having a first data value-to-luminanceresponse to a viewer, the image perceived by the viewer through spatialaveraging is substantially the same as the original image, and such thatwhen the modified image is displayed in a second scenario using adisplay panel having a second data value-to-luminance response to aviewer, different to the first data value-to-luminance response, theimage perceived by the viewer through spatial averaging is different tothe original image, and wherein the first and second configurations ofliquid crystal are arranged to provide the display panel withsubstantially the first and second data value-to-luminance responsesrespectively for viewing angles outside the narrow range, and are botharranged to provide substantially the first data value-to-luminanceresponse for viewing angles inside the narrow range.
 2. A display deviceas claimed in claim 1, wherein the second data value-to-luminanceresponse is a substantially non-linear data value-to-luminance response.3. A display device as claimed in claim 1, wherein the first datavalue-to-luminance response is a substantially linear datavalue-to-luminance response.
 4. A display device as claimed in claim 1,wherein the liquid crystal in the first configuration comprises aplurality of lateral regions each having one of at least two differentarrangements of liquid crystal, and wherein the first configurationregions are adapted such that light travelling at an angle outside thenarrow range of angles passes through at least two regions havingdifferent arrangements of liquid crystal so as to have the first datavalue-to-luminance response.
 5. A display device as claimed in claim 1,wherein the liquid crystal in the second configuration comprises aplurality of lateral regions each having one of at least two differentarrangements of liquid crystal, and wherein the second configurationregions are adapted such that light travelling at an angle outside thenarrow range of angles has the second data value-to-luminance response.6. A display device as claimed in claim 1, wherein the first and secondconfigurations are twisted nematic configurations.
 7. A display deviceas claimed in claim 1, comprising at least one patterned alignment layerfor producing the first and second configurations.
 8. A display deviceas claimed in claim 7, wherein the circuitry is operable to apply anelectric field to change the alignment properties of the at least onealignment layer to switch the liquid crystal between the first andsecond configurations.
 9. A display device as claimed in claim 1,wherein the circuitry is operable to apply an electric field acrossand/or in the plane of the liquid crystal to switch the liquid crystalbetween the first and second configurations.
 10. A display device asclaimed in claim 9, wherein the circuitry is operable to apply fringeelectric fields.
 11. A display device as claimed in claim 1, wherein theoriginal image is substantially hidden in the image perceived in thesecond scenario.
 12. A display device as claimed in claim 1, wherein atleast some of the data values are modified in dependence upon a maskingimage.
 13. A display device as claimed in claim 12, wherein each datavalue is modified in dependence upon the data value at a correspondingposition of the masking image.
 14. A display device as claimed in claim12, wherein the image perceived in the second scenario resembles atleast to some extent the masking image.
 15. A display device as claimedin claim 12, wherein the original image is substantially hidden in theimage perceived in the second scenario, and wherein the masking image issuch as to provide a high degree of visually confusing information inthe second scenario.
 16. A display device as claimed in claim 15,wherein the masking image comprises a chequerboard pattern or a patternof text or a logo.
 17. A display device as claimed in claim 12,different masking images are used in different time frames.
 18. Adisplay device as claimed in claim 1, wherein at least some of the datavalues are modified in dependence upon a masking parameter.
 19. Adisplay device as claimed in claim 18, wherein the degree ofmodification is determined at least in part by the masking parameter.20. A display device as claimed in claim 1, wherein the data values aremodified such that localised groups of displayed image elements areperceived in the first scenario through spatial averaging to havesubstantially the same overall luminance as those image elements wouldhave done without such modification.
 21. A display device as claimed inclaim 20, wherein at least some of the data values are modified independence upon a masking image, and wherein the degree of modificationfor each image element in a group is determined in dependence upon thedata value at a position in the masking image corresponding to the imageelement.
 22. A display device as claimed in claim 20, wherein, if anymodification is to be performed for a group, the data value of at leastone image element in the group is increased while the data value of atleast one other image element in the group is decreased.
 23. A displaydevice as claimed in claim 22, wherein the amount of increase issubstantially the same as the decrease.
 24. A display device as claimedin claim 22, wherein the amount of increase relative to the amount ofdecrease is determined in dependence upon the first datavalue-to-luminance response.
 25. A display device as claimed in claim22, wherein the image elements designated for increase and decrease areswapped in different time frames.
 26. A display device as claimed inclaim 22, wherein at least some of the data values are modified independence upon a masking image, and wherein the degree of modificationfor each image element in a group is determined in dependence upon thedata value at a position in the masking image corresponding to the imageelement, and wherein for each of the at least one image element anamount related to the corresponding respective masking image data valueis added to the image element data value, and for each of the at leastother one image element an amount related to the correspondingrespective masking image data value is subtracted from the image elementdata value.
 27. A display device as claimed in claim 26, wherein theamount is equal to the corresponding masking image data value.
 28. Adisplay device as claimed in claim 26, wherein the amount is determinedin dependence upon the difference between the image data value and themaximum or minimum data value, whichever is closer.
 29. A display deviceas claimed in claim 28, wherein the amount is proportional to thedifference multiplied by the corresponding masking image data value. 30.A display device as claimed in claim 20, wherein each group comprisestwo image elements.
 31. A display device as claimed in claim 20, whereinthe data values of at least some of the image elements are averaged in agroup.
 32. A display device as claimed in claim 20, wherein the datavalues of at least some of the masking image elements are averaged in agroup.
 33. A display device as claimed in claim 20, wherein imageelements having corresponding modifications performed on them arearranged in lines of the image.
 34. A display device as claimed in claim20, wherein image elements having corresponding modifications performedon them are arranged in columns of the image.
 35. A display device asclaimed in claim 20, wherein image elements having correspondingmodifications performed on them are arranged in a chequerboard patternor a pattern of text or a logo.
 36. A display device as claimed in claim1, wherein it is ensured that a modified data value does not falloutside the normal range of data values allowed.
 37. A display device asclaimed in claim 36, wherein at least some of the data values aremodified in dependence upon a masking image, and wherein the degree ofmodification for each image element in a group is determined independence upon the data value at a position in the masking imagecorresponding to the image element, and wherein the data value range ofthe image is compressed before modification.
 38. A display device asclaimed in claim 36, wherein the data value range of the masking imageis compressed before modification.
 39. A display device as claimed inclaim 1, wherein only the data values of the image elements in asub-portion of the image are so modified.
 40. A display device asclaimed in claim 1, wherein each image element relates to a plurality ofcolour components corresponding to a pixel of the display device.
 41. Adisplay device as claimed in claim 1, wherein each image element relatesto a single colour component corresponding to a sub-pixel of the displaydevice.
 42. A display device as claimed in claim 1, wherein the vieweris at least a predetermined distance away from the display device.
 43. Anon-transitory computer readable storage medium storing an operatingprogram which, when loaded into an apparatus, causes the apparatus tobecome apparatus as claimed in claim 1.